Dark trace cathode-ray tube



De- 1 1953 A. E. BEcKERs 2,661,437

` DARK TRACE CTHODE-RY TUBE Filed June 19, 1951 nanna uurpn oooocooo90000000.0000..

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' ALBERT E. Blawgs ATTORNEY Patented Dec. 1, 1953 DARK TRACE CATHODE-RAYTUBE Albert E. Beckers, East Orange, N. J., assigner to National UnionRadio Corporation, Orange, N. J., a corporation of Delaware ApplicationJune 19, 1951, Serial No. 232,319

(Cl. S13- 71) 14 claims. l

This invention relates to electron trace tubes, and more especially itrelates to tubes of the socalled dark trace or tenebrescent kind.

As is Well-known, dark trace tubes employ a screen consisting of a layerof crystals which exhibit tenebrescence or formation of dark colorcenters when bombarded by relatively high velocity electrons from anelectron gun. Examples of such crystal screens are those oi' the alkalihalide type formed of crystals of an alkali halide or mixture of suchhalides, for example potassium iodide, potassium chloride and the like.

One of the disadvantages of the dark trace tube as compared with tubesusing a iluorescing screen, is the fact that once an opacity center isformed it tends to remain dark or opaque. and special procedures arerequired to restore the individual darkened areas to their normal lightconduction or light reflection condition. Heretofore two methods havebeen used to effect the erasure of the dark trace record. One suchmethod employs a source of heat adjacent the dark trace screen, but thatmethod is relatively slow in operation and requires a period of severalminutes to restore the screen to its normal condition. The second methoduses an electron beam to effect the erasure. 'I'hus the customary way ofeffecting erasure by an electron beam is to use the same electron gunthat is used for the dark trace recording, and to greatly increase thebeam current for erasure as compared with the beam current forrecording. Because of the extraordinarily high beam current, the lifeand uniformity of the gun are severely curtailed. Thus the usual cathoderay gun employs a cathode emission surface which is very small in area,for example, less than one millimeter and only the actual centralportion of that area is useful in developing the cathode ray or electronbeam. Therefore in order to secure uniformity of beam current for agiven control voltage and in order to provide a reasonably long life, itis necessary to protect the cathode during recording and at other times,against excessive current drain. An average emission current of not morethan 100 microamperes has been found admissible. Since the cathode issubjected to its greatest strain or load during erasure it has beenproposed heretofore to employ a second gun siznilar to the recording orwriting gun but which comes into operation only during erasure. Howeverthe use of a separate erasure gun introduces a number of disadvantages.First the additional space needed for location of the separate gun is asa practical matter not available in the customary cathode ray tubeconstruction and a separate tube neck must be used to house the separategun. This requires separate deflection and beam focussing coils on thetwo necks with the likelihood of undesirable mutual interaction.Secondly the provision of two separate necks complicates the manufactureand assembly and greatly increases the cost of the finished tube. Thetube becomes unwieldy to handle and requires extensive change in themechanical and electrical design of the associated equipment with whichit is to work.

Therefore it is one of the principal objects of the present invention toprovide an improved arrangement for enabling the record on the screen ofa dark trace tube to be erased without reducing the useful life oruniformity of operation of the tube and Without greatly increasing thecost of manufacture.

Another object is to provide an improved construction of dark trace tubewherein the erasure can be effected with rapidity and wherein theerasing action does not introduce any undesired coloring effect on thescreen.

Another object is to provide an erasing arrangement for dark tracetubes. which does not require the action of beam focussing and beamdeilecting during the erasure.

A feature of the invention relates to a dark trace cathode ray tubehaving a novel construction and location of electron nood gun foreffecting erasure of a dark trace record.

Another feature relates to an improved dark trace cathode ray tubehaving mounted relatively close to the dark trace screen. a specialerasure iilamentary cathode for subjecting the screen to a flood oferasing electrons uniformly over its entire area and Without requiringan abnormally high voltage gradient between the filament and screen.

Another feature relates to a novel construction and mounting arrangementfor an erasing flood gun for dark trace cathode ray tubes.

A further feature relates to the novel organization, arrangement andrelative location of parts which cooperate to provide an improvedcathode ray tube of the dark trace recording type.

Other features and advantages not particularly enumerated, will becomeapparent after a consideration of the following detailed descriptionsand the appended claims.

In the drawing, which shows by way of example one preferred embodiment,

Fig. 1 is a plan View. partly sectional, of a dark trace cathode raytube according to the invention.

Fig. 2 is a magnified sectional view of part of Fig. 1.

Fig. 3 is a perspective view of one half of the screen with its mountingarrangement and associated erasure flooding filament.

Fig. 4 is a front View of the erasure flooding filament itself.

Fig. 5 is a schematic diagram explanatory of certain features of theinvention.

Referring to Fig. l, there is indicated by the numeral .LILanywell-known form of cathode ray tube envelope, comprising the elongatedneck portion II joined to the flared bulb portion I2, which is closedoff by the transparent end wall I3. Attached to the neck II is anywell-known plug-in base I4 carrying the usueilcontactprongs I5. Thevarious lead-in wires .andelectrode support wires (not shown) are sealedthrough the stem or header (not shown) which closes off `the neck I Iand through which the bulb can be evacuated in known manner. Mountedywithin the neck is any well-known form of electron gun comprising forexample the .electron-emitting cathode I6, control grid I1, rstaccelerating anode I8, second accelerating anode I9. These electrodes,for simplicity of showing, are indicated schematically in Fig. l, sincetheir actual structure is well-known. Sufice it to say that by suitableheating of the cathode, and by suitable energization of the anodes bydirect current voltage, the electrons from the cathode are formed into abeam which can be focussed upon the recording screen by the conventionalfocussing yoke 20. If desired, the inner surface of the bulb I2 can becoated with a conductive material which can be connected to the secondanode potential for purposes well-known in the art. Surrounding the neckare the usual magnetic yokes 2 I, 22, for subjecting the focussed beamto the desired pointby-point scanning motion over the screen.

Mounted at the end of the tube adjacent the wall I3 is the dark tracerecording screen and erasure assembly according to the invention. Thescreen proper, comprises a transparent sheet or support 23 (Figs. 2 and3) of mica or thin glass which is coated on the side facing the electrongun with a layer 24 of tenebrescent material such as potassium iodidecrystals, potassium chloride crystals, or crystals of other tenebrescenthalide or halides. This tenebrescent layer is coated with, for exampleby having evaporated thereon, a Vlayer 25 of aluminum of sufficientthinness as to be transparent to the electron beam 2B which is focussedon layer 24.

The screen comprising elements `23, 24, 25, is mounted in an annularmetal supporting frame comprising the metal ring 21 having an integralnange 28, and the said screen is pressed against the ring 2T through theintermediary of a compressible metal ring 29 and another metal ring 30which ls fastened to the flange 28. The ring 30 has a pair of oppositelyextending flanges 3l, 32. Flange 3l acts as a retainer for thecompressible metal ring 29, and the flange 32 is for attaching to theflange 28. This latter fastening can be effected in any suitable way,for example by a plurality of straps 33, which can be clinched aroundthe adjacent edges of flanges 28, 32, after the ring 3U has been pressedagainst the compressible ring 29 to insure good, but somewhat yieldable,electrical contact between the supporting frame and the coating 25.

In accordance with one feature of the invention, the compressible ring29 may be formed from a sheet or ribbon of aluminum or similar metal ofabout 0.00025" thickness, and which after being rolled up longitudinallyis shaped to form an annular washer which extends around the entiremargin of the coating 25 and is capable of being compressed to providethe requisite resilient; pressure to hold the screen in place whilemaking good electrical contact to the said coating, when the ring 30 isadjusted in place. Electrical contact can be made to the coating 25 by a4 fine wire 34 sealed through the wall of bulb I2 and provided with anywell-,knownexternal contact button-'3 5.

Welded or otherwise fastened to the ring 30 1s aseries of metal tabs 3E,each of which carries an individual glass bead 31 and with each beadcarrying an individual lament supporting hook 38 which hooks aretherefore insulated from ring 30. Strung A.backend forth in zig-zagfashion between the successive hooks and in substantial coplanar form,is a fine wire electron emissive filament 39 :for example of 1%thoriated tungsten wire oi' approximately 0.0016" diameter. Thisfilament constitutes the cathode of the erasure flood gun and is mountedby means of the hooks 38 re1- atively close to the dark trace screen,for example at a distance of one inch. The two ends of the filament arewelded to respective lead-"ln wires 40, sealed through the bulb andprovided with conventional contact buttons 4I on the external surface ofthe bulb, Thus the flood ,gun filament can be connected to a suitablesource of current to raise it to the requisite electron-emittingtemperature for example 2000" K, and at the same time a suitable directcurrent potential. for example about 500 volts can be applied to thedark trace screen through contact button 35.

In one tube that was found to have the desired recording and erasurecharacteristics, the screen had a diameter of 6.5 inches and the floodgun erasure filament was stretched in a zig-zag form as shown in Fig. 4,so as to be substantially uniformly effective over the entire screenarea. This filament during the erasure interval only, was connected to asource of approximately 450 volts A. C. to produce a filament current ofapproximately 395 milliamperes, thus raising it to a temperature ofapproximately 2000 K. At the same time a direct current voltage of about500 volts with respect to the filament center, was applied to the screenthrough contact 35. As a result, the screen was bombarded by an electronflow of 150 milliamperes and all the previously recorded dark traceareas were uniformly and completely restored to their normal stateWithin 7 seconds. It was found that when the filament was heated butwithout applying a Voltage to the screen, while erasure took place itrequired a period of from to 120 seconds.

It will be understood that the filament 39 can be of sufficient thinnessso as not to cast any appreciable electron shadow on the screen vduringthe Writing or recording operation. If however such a shadow should tendto appear, I have found that it can be avoided by applying a relativelysmall positive potential to the filament during the recording. Thus inthe particular tube mentioned above barely perceptible shadows existedwhen there was applied to the filament a positive potential of 50 voltswith respect to the screen and second anode I9.

As shown in Fig. 5, the width W of the electron flow from each one ofthe parallel strands of the filament to the screen is determined `by thedistance d" between the filament and screen. by the well-known workfunction of the cathode material expressed in E volts, and by thevoltage U between the cathode and screen. Therefore, the width when itis heated for erasure it uniformly sprays the entire area of the screen.

Various changes and modifications may be made therein without departingfrom the spirit and scope of the invention.

What is claimed is:

1. A cathode ray tube having an enclosing envelope containing a screenwhich becomes tenebrescent in response to impinging electrons in a beamof one range of intensity and which becomes non-tenebrescent in responseto impinging electrons in a different range of intensity, a rst electronemitter for developing tenebrescence in said screen, a second electronemitter closely adjacent the screen for erasing said tenebrescence, andlead-in means for said second emitter for supplyingr heating currenttherethrough.

2. A cathode ray tube having an enclosing envelope containing a screenwhich becomes tenebrescent in response to impinging electrons in a beambelow a certain intensity, a first electron emitter for developing s( idbeam, a second electron emitter located between the first emitter andthe screen so as to be traversed by the electrons from the firstemitter, and means to apply a heating current through to said secondemitter to simultaneously flood substantially the entire area of saidscreen with electrons to erase the tenebrescent record produced thereonby said beam.

3. A cathode ray tube having an enclosing envelope containing a screenwhich becomes tenebrescent in response to an impinging recordingelectron beam, and means to erase said tenebrescence comprising anelectron flood gun consisting solely of a heatable fine wire electronemitter mounted closely adjacent to said screen, and means to supplyheating current through said filament to simultaneously flood saidscreen with electrons of markedly higher beam intensity than the firstbeam.

4. A cathode ray tube having an enclosing envelope containing a screenwhich becomes tenebrescent in response to an impinging recordingelectron beam, an electron gun for developing said recording electronbeam, means to move said beam in a desired scanning pattern over saidscreen, and another electron emitter mounted closely adjacent to saidscreen, lead-in means for supplying said other emitter with heatingcurrent for producing simultaneously a multiplicity of adjacent electronbeams to flood said screen with electrons of much higher beam intensitythan the first beam to erase the tenebrescent record on said screen.

5. A cathode ray tube having an enclosing envelope containing .a halidecrystal screen of the type which becomes tenebrescent in response to animpinging electron beam, an open-work electron emitter mounted closelyadjacent to said screen, and lead-in means to supply heating current tosaid emitter to flood substantially the entire screen at much higherintensity to erase a previously recorded tenebrescent condition on saidscreen.

6. A cathode ray tube having an enclosing envelope containing a halidecrystal screen of the type which becomes tenebrescent in response to animpinging electron beam, an erasing flood gun including a foraminouselectron emitting cathode mounted closely adjacent to said screen, andlead-in means for supplying heating current to said foraminous cathode.

7. A cathode ray tube according to claim 6 in which said cathode isfilamentary and with a multiplicity of spaced strands, the ends of thefilament being connected to contact terminals for connection to a sourceof heating current.

8. A cathode ray tube according to claim 6 in which said cathode isfllamentary and consists of a flne Wire strung in zig-zag form adjacentsaid screen.

9. In a cathode ray tube of the kind described, a halide crystal screenhaving an electron transparent metal coating, a metal frame forsupporting said screen Within said tube, and an open- Work filamentarycathode insulatingly carried by said frame closely adjacent to thescreen, and lead-in means to supply heating current to said open workcathode for flooding the screen with erasing electrons to erase apreviously recorded tenebrescent condition on said screen.

10. A cathode ray tube according to claim 9 in which said frame isprovided with an external connection for applying a voltage differencebetween the screen and cathode, and said lamentary cathode is providedwith external terminals for applying heating current thereto.

1l. A cathode ray tube comprising a recording screen to be scanned by anelectron beam, said screen having an electron transparent metal coatingand a frame for supporting said screen in electrical contact with saidframe, said frame comprising a pair of metal rings and an elasticallycompressible metal member arranged to be clamped between said rings andin conductive Contact with said coating.

12. A cathode ray tube according to claim 11 in which said yieldablemetal member comprises a sheet of thin metal which has been rolled onitself longitudinally and is formed to washer shape.

13. A cathode ray tube according to claim 12 in which said yieldablemetal member is formed of aluminum sheeting.

14. A recording screen and erasure assembly for dark trace cathode raytubes, comprising a light transparent plate having an electrontransparent coating and a layer of tenebrescent halide crystals betweensaid plate and coating, a metal frame for supporting said plate within acathode ray tube envelope, and a multi-strand llamentary cathode carriedby said frame in predetermined closely spaced relation to said plate,the said strands being spaced apart to produce a series of electronbeams impinging on said screen, the width W of each beam with respect tothe spacing of said cathode from said screen being deflned by theformula where E is the work function of the cathode material, d is thespacing between the screen and said cathode, and U is the voltagebetween the cathode and screen.

ALBERT E. BECKERS.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,228,388 Farnsworth Jan. 14, 1941 2,239,887 Ferrant Apr. 29,1941 2,259,507 Iams Oct. 21, 1941 2,276,359 Von Ardenne Mar. 17, 19422,402,762 Leverenz June 25, 1946 2,438,668 Koch et al Mar. 30, 19482,533,381 Levy et al Dec. 12, 1950 2,582,843 Moore Jan. l5, 1952

